A conventional intrinsic photoconductive detector comprises a body of photosensitive material having a pair of spaced bias contacts. Incident radiation, having energy greater than or equal to the bandgap of the photosensitive material, is absorbed and electron-hole pairs are generated resulting in an increase in conductivity.
It is known that the sensitivity of a detector is enhanced by photoconductive gain. Photoconductive gain is typically a result of the difference in the times that electrons and holes spend within the photoconductive materials. For example, the mobility for electrons in InGaAs at room temperature is about 40 times greater than the mobility for holes in that material. When a photon is absorbed and an electron-hole pair is generated, many electrons are injected by the negatively biased contact and traverse the photoconductive material during the time the photogenerated hole exists in that material. The photoconductive gain, therefore, is roughly equal to the number of electrons which cross the photoconductive material for each photon absorbed.
Equally important of the performance of a detector is its bandwidth which effectively is a measure of how fast the device responds, i.e. how many bits of information it can detect per second. The bandwidth is determined by the time that a photogenerated hole spends in the photoconductive material. The longer the time that the hole remains in the photoconductive material, the smaller the bandwidth, i.e. the slower the response of the device.
In the area of high-speed optical communications systems, where data rates are in the gigabit/second range, there has been a significant interest in lateral photoconductors because of their bandwidth and photoconductive gain capabilities. By lateral photoconductor is meant a device wherein light is incident upon a thin layer of photosensitive material in a direction perpendicular to that of the current flow. Also, lateral photoconductors are ideal for monolithic integrated photoreceiver applications because their structures are compatible with those of field-effect transistors.
Since these devices conventionally employ materials with high mobility ratios to realize a higher gain, it becomes necessary to provide a mechanism by which the time the hole spends in the active region is reduced for a larger bandwidth. One approach has been to form a detector comprising a substrate, typically of the N+ conductivity type, and thereover an active region, typically undoped or lightly doped. Anodes and cathodes are interdigitated on the upper surface of the active region and the anode-to-cathode spacing must be sufficiently small to provide that the holes are swept out of the photosensitive area at least as fast as the data rate. However, this means that about 30 to 50 percent of the incident light will be obscured by the interdigitated anodes and cathodes on the detector surface thus, decreasing the incoming signal. Also, with both electrodes on one surface there exists a nonuniform electric field across the photosensitive area which has the unwanted effect of decreasing the bandwidth and creating a spatially non-uniform gain. Further, because much of the incident light is absorbed close to the anodes and cathodes, up to 50 percent of the gain otherwise realizable is lost.
Other efforts to accommodate data rates in the gigabit/second range include variations on the interdigitated approach. For example, one scheme utilizes a reverse biased p-n junction behind the photoconductive layer to remove the holes from the photoconductive material. This device is characterized, however, by reduced gain and increased generation-recombination noise.
Alternatively, some recent efforts have been directed to vertical photoconductors. In FIG. 1 a prior art photoconductive detector 10 comprises a substrate 12 typically of N+ conductivity type, an active region 14 of N- conductivity type and a thin contact window 16 of N+ conductivity type. The thickness of the active region 14 is generally about 2-3 .mu.m. A ring-shaped metal contact 20 defines an opening 18 through which light enters which is a larger area than in the lateral device. The N+ substrate 12 and N+ contact window 16 serve as ohmic contacts to the active region 14 across its entire width. In theory this should provide uniform vertical fields across the active region 14. In long wavelength (1.0-1.6 .mu.m) applications the active region 14 is typically of InGaAs and, for considerations relating to noise and resistance levels, the contact window 16 is of the same material. This provides however, that the contact window 16 must be relatively thin, i.e. about 0.5 to 1.0 .mu.m so that most of the light will not be absorbed by the contact window 16. At these thicknesses, a lateral electric field exists across the contact window 16 and into the active region 14 due to the location of the ring contact 20. Thus, the electron and hole trajectories become lateral and the distances traveled become greater than the active region thickness thereby increasing the gain but greatly reducing the bandwidth. Also, since the substrate 12 is typically of InP, holes traveling towards the substrate 12 may be caught in charge traps known to exist for minority carriers at this InGaAs/InP interface 24. This also results in increased gain but can significantly reduce the bandwidth.
A photoconductive detector suitable for high speed operation with high gain, reduced noise and trapping and straightforward adaptability to monolithic integration had been sought.